Pupil and lens

Strictly speaking, it is not the pupil that changes its diameter. What looks to the observer as if the pupil is getting smaller or larger is actually the change in the iris, which opens and closes much like the shutter of a camera. Depending on the light conditions, this reduces or enlarges the pupil. This is done so that only as much light enters the eye as the retina can process. If it is bright, the incoming light is focused by reducing the size of the hole in the centre of the iris. In the dark, on the other hand, the retina needs more light. The pupil dilates to let more light in.

This is done by two iris muscles: the sphincter pupillae and its counterpart, the dilator pupillae. They are arranged in a circular, resp. radial form at the back of the iris and are in contact with the retina. This means that they can always react immediately to the light conditions and open or narrow the pupil.

But drugs or sexual arousal can also cause the black area in the centre of the iris to open up. For example, studies have shown that faces with dilated pupils appear more attractive than those with narrow pupils. This is why women used to drip the juice of belladonna into their eyes when they wanted to "make eyes at someone". This is because the alkaloid atropine from belladonna paralyses the muscle responsible for closing the pupil. This is where the scientific name of belladonna comes from: Atropa belladonna, derived from "beautiful woman".

Although “iris” is Greek for "rainbow", we usually only have one definite eye colour. Depending on the number of melanin pigments, the eyes are blue or brown. The colour is created by the scattering of light on the collagen fibres of the iris. The more pigments there are, the darker the eye colour will appear.

The lens of the eye is located directly behind the iris and is not visible from the outside. It is convexly curved on both sides and can thus focus the light entering through the pupil in such a way that a sharp image is formed on the retina at the back of the eye. Although the lens is made of collagen fibres, which are also found in the skin, bones, fingernails and hair, it is completely transparent due to a special arrangement of its fibres.

The structures of the eye lens remain almost unchanged over a lifetime. They are already created in the fifth week of pregnancy. Two special protective proteins ensure that the proteins of the lens do not clump together and thus cloud the vision.

The lens provides the near and far focus of the eye. This process is called accommodation. Around the lens are the zonula fibres that attach it to the ring-shaped ciliary muscle. When this muscle contracts, the zonula fibres slacken, causing the lens to focus to near range. When the muscle relaxes, the lens of the eye adjusts to distant vision.

With increasing age, the lens of the eye can become cloudy, severely limiting the ability to see. This clouding of the lens is called a cataract. It can be corrected by an surgery in which the clouded lens is replaced by an artificial lens through an incision at the border between the conjunctiva and the cornea.